The present invention relates to an exposure method for semiconductor wafers, and, more particularly, to block exposure. This invention also relates to a mask for exposure, a block exposure method, an exposure data generating method, a mask preparing data generation method and a storage medium.
As recent LSI technology seeks out larger scale integration and further miniaturization, exposure techniques for improving the speed and precision of exposing an LSI pattern are needed. Block exposure is one known exposure technique.
In a block exposure, some common portions in an LSI pattern are extracted as a block and are formed on a stencil mask or a block mask, and the cross-sectional shape of a beam is formed (shaped) by the block based on the patterns of the common portions. This scheme permits a plurality of patterns to be exposed in a single shot, which shortens the exposure time and thus shortens the time of processing a wafer.
Each stencil mask deteriorates as the number of shots increases. In accordance with the deterioration, replacement of degraded stencil masks becomes necessary. Such work prolongs the processing of wafers. To avoid this problem, an improvement in the durability of stencil masks is needed.
FIG. 1 presents a schematic diagram of an electron beam exposure apparatus that performs block exposure. The exposure apparatus 10 has a plate 11 and a stencil mask 13. A rectangular window 12 having a predetermined area is formed in the plate 11. As shown in FIG. 2, the stencil mask 13 has a plurality of block areas 14 and rectangular windows 15 formed.
A block pattern, including some common portions in the layout pattern of a semiconductor device, is formed in each block area 14. Each block pattern includes plural types of patterns and is mounted on the associated block area 14, which is why such a block pattern is called a "mounting block".
A first electromagnetic deflector 17 of the exposure apparatus 10 deflects an electronic beam, which is emitted from a beam source 16 and passes through the rectangular window 12 in the plate 11, so that the beam is selectively irradiated on one block area 14 on the stencil mask 13. A second electromagnetic deflector 18 of the exposure apparatus 10 deflects a formed beam, which has passed through a block area 14 on the stencil mask 13, to expose the extracted layout pattern on a wafer 19.
Since a plurality of patterns are formed in each block area 14, a plurality of patterns can be exposed at a time by a single shot of a beam. Therefore, the exposure time is shortened by mounting preferentially those block patterns on the stencil mask that are likely to be frequently transferred on the wafer 19.
Since a plurality of layout patterns are exposed on the wafer 19 by a beam, which has passed through a single block area 14, layout patterns of the same shape can be formed on the wafer 19 with a high precision. Those features therefore contribute to shortening the exposure time per wafer.
Mounted on the stencil mask 13 used for the exposure of a semiconductor device like a DRAM are a block of layout patterns that are repeatedly used, like those for memory cells, and a block of layout patterns that are less likely to be repeatedly used, like those for input/output circuits. In exposing one chip of a semiconductor device, the number of times a block extracted from memory cells is used, i.e., the number of transfers of the block, is several hundred to several thousand times that of blocks extracted from patterns other than memory cells.
Since a plurality of chips are generally formed on a single wafer 19, the number of transfers of a block extracted from memory cells per wafer is equal to the number of transfers per chip multiplied by the number of chips to be formed. The transfer number of a memory cell block therefore is considerably greater than that of other types of blocks.
In exposing the wafer 19 using the stencil mask 13, an impurity like carbon may adhere to the stencil mask 13. Carbon is included in a resist coating on the wafer 19. When the resist coating is heated by the electron beam, the carbon is scattered and may stick to the stencil mask 13.
The adhered impurity reduces the transmittance of the beam passing through the block. The greater the number of transfers of a block becomes, the lower the transmittance through the block becomes. This may result in insufficient exposure of a layout pattern or may blur the edge of a layout pattern.
The stencil mask 13 is heated by beam irradiation, and the amount of heat increases as the number of beam irradiations or the number of transfers of a block increases. Further, the heat makes the breakage of block patterns from the stencil mask 13 easier.
As apparent from the above, repeatedly used blocks suffer a lower transmittance and a higher defect rate than blocks used fewer times. The greater the number of transfers of a block is, therefore, the shorter the remaining life of the stencil mask 13 and the lower the durability of the stencil mask 13 becomes.
The durability of the stencil mask 13 influences the fabrication time for a semiconductor device. Some blocks are degraded considerably from repetitive usage, while other blocks are still sufficiently usable, which causes frequent stopping of exposure for replacement of the stencil mask 13 or frequent position adjustment of the stencil mask after replacement. This reduces the number of wafers exposed over a given period of time and lengthens the processing time for each wafer 19. This leads to an increased processing cost of semiconductor integrated circuit devices.